1. Field of the Invention
The present invention relates to a cell disassembly unit for returning signals in ATM cell format, which have been converted from the signals in an existing STM network, back to the signals in the existing STM network again in the transmission and switching equipment, based on the ATM transmission system, which is a key technology of B-ISDN.
2. Description of Related Art
The cell assembly unit is equipment for assembling the time slots of a connection (call, path) which subscribers use in an STM (Synchronous Transfer Mode) network into a cell for each connection. A cell disassembly unit, on the other hand, is equipment which performs an operation the opposite of a cell assembly unit. In other words, a cell disassembly unit is equipment for disassembling a cell in ATM (Asynchronous Transfer Mode) into time slots (bytes) so as to convert into signals of an STM network. A cell disassembly unit constitutes a part of CLAD (Cell Assembly and Disassembly).
In an ATM network, however, cells may be lost for such reasons as a miss in the transfer of cells due to bit errors or discarding of cells at a switching section, and when a cell generated by a cell assembly unit is lost, the time required for data seems to decrease if cells to arrive are simply disassembled in sequence, since the original data (STM signals) is data with a fixed speed.
For example, normal voice data is sampled at 8 kHz when digitized. Since 1 sample is 1 byte, 8000 bytes of voice data are generated in one second, which means 80000 bytes in 10 seconds.
Since 8000 bytes of voice data are generated per second at this fixed speed, the side which reproduces voice must process data at the speed of 8000 bytes per second. If the sending side (generating side) transmits data for 10 seconds (80000 bytes), the reproducing side also completes reproduction for 10 seconds. In this case, there is no difference in the processing time between the generating side and the reproducing side.
Now it is assumed that 10 seconds of data, that is, 80000 bytes of data for example, generated by the generating side is partially lost during transmission, and only 72000 bytes of data arrive at the reproducing side. The reproducing side, which does not know that data is missing, sequentially processes the arrived signals at 8000 bytes per second. Here the reproducing time takes only 9 seconds. Generation time which originally took 10 seconds is reproduced in 9 seconds, where the time required for the data seems to have decreased.
If the lost data is not compensated for, then the time difference between the generating side and the reproducing side increases infinitely, and theoretically the data is reproduced faster than the time it took for generation. This is, of course, impossible, so the system breaks down if such a time difference increases.
When cell loss occurs in an ATM network, if the receiving side simply dissembles the arrived cells, then the position of the data on the time axis changes, and data space seems to decrease. For example, it is assumed that data sent at the fifth second is lost during the transmission in the above mentioned case. In this case, at the reproducing side, the data at the sixth second comes next to the data at the fourth second. Therefore at the reproducing side, data at the sixth second and later each shift 1 second in the data position on the time axis.